Many winch applications have a need for two-speed operation to maximize the efficiency of the winch on a crane or other application. This is because much of a crane's time is spent moving light loads, which requires a small fraction of the winch system capacity. A single-speed winch circuit requires that the prime mover (engine) be sized to handle the peak horsepower requirements of the heavy loads. This results in a system which is used only to partial capacity much of the time. This is wasteful of the operator's time and system horsepower.
A way of minimizing the inefficiencies described above is to provide a two-speed hydraulic motor for the winch, with some means of shifting speeds by directing oil (working fluid) to both motor elements, or one element only and recirculating the second element in a loop. This valving mechanism has the requirement that during valve spool transition from one extreme to the other, it neither blocks the outlet of a motor element, causing a blocked-flow condition, nor allows a cross-port flow path, which permits the total motor displacement to circulate from inlet to outlet ports in an unrestricted path. A momentary cross-port flow will cause the motor to be effectively in a momentary free-wheeling condition where it can neither provide torque to continue raising the load nor provide a braking action in which the motor tries to pump against a brake valve. Obviously, this condition is undesirable because it provides the potential for dropping the load some distance.
Currently, other valves available employ secondary logic spools and check valves which are separate from the main valve shift spool. This causes most designs to be unduly expensive and bulky.